Apparatus for the manufacture of chrome powder



Oct. 27, 1970 v M. GOHIN ETAL 3,536,309

APPARATUS FOR THE MANUFACTURE OF CHROME POWDER Filed Feb. 26, 1968 Fig.2

Fig.3

Fig.1

3,536,309 APPARATUS FOR THE MANUFACTURE OF CHROME POWDER Gilles M. Gohinand Andre R. Hivert, Pontoise, France,

assignors to Oflice National dEtudes et de Recherches Aerospatiales,Chatillon-sous-Bagneux, France Filed Feb. 26, 1968, Ser. No. 715,114Claims priority, applic9atigr719France, Feb. 28, 1967,

Int. C1. (5221; 39/00 US. Cl. 266-24 9 Claims ABSTRACT OF THE DISCLOSUREThe invention relates to a process and an apparatus for the manufactureof chromium powder.

It has already been proposed to manufacture chromium powder bysubjecting chromium oxide powder to the action of magnesium in vapourform. In this way, starting from fine chromium oxide powder, sometimescalled chromium green, chromium powder "of a previously unobtainabledegree of fineness and of great purity is obtained.

The object of this invention is to extend the process on an industrialscale, i.e. to make it possible to treat in a single operation masses ofchromium oxide of suflicient bulkiness for the cost to permit numerousand wide applications.

A particular object of the invention is to provide a process and anapparatus for obtaining in a single operation chromium powder in severaldozen kilograms or even more in spite of the highly exothermic characterof the reaction.

Another object of the invention is to provide a process and an apparatuswhereby an exothermic reaction can be controlled satisfactorily withoutprolonging excessively the reaction time.

A general object of the invention is to provide a process and anapparatus for obtaining chromium powder of a much greater degree offineness than that of ordinary industrial powder and at a competitivecost.

The process involves using magnesium in the most available form, i.e. inthe form of ingots, and disposing the powder to be treated in the samereceptacle as the ingot or ingots, so that the working of the process isparticularly simple.

The invention comprises steps that enable violence of the reaction to beeffectively counteracted. This violence is liable to occur because ofthe exothermic character of the reaction. By these steps the temperatureof the whole is limited to the desired value so that the vaporisation ofthe magnesium is not acceleratedbeyond what is necessary.

According to the invention, for the industrial treatment of chromiumoxide powder with magnesium in vapour form in order to obtain chromiumpowder, an industrial quantity of chromium oxide powder is divided intomasses between one and about 10 kilograms, magnesium in in- UnitcdStates Patent 3,536,309 Patented Oct. 27, 1970 got form is embedded ineach of these masses, which are thermally insulated from each other, andthe industrial quantity is heated in the same enclosure and in a singletreatment phase.

Thus each portion of the mass is brought into the optimum reactionconditions without being subjected excessively to the influence of theheat liberated by the other portions simultaneously undergoing thereaction. According to the invention then, the process resulting in theproduction of chromium oxide powder is influenced by dividing thetreated mass.

The invention makes it possible to take advantage simultaneously of thebulk of the mass treated in one operation, with the resulting economicadvantages, and also the strictness of the control to which each of theportions can be subjected so that the reaction may proceed under optimumconditions, with, as a result, a complete or practically completeconversion of the chromium oxide into chromium of exceptionally fineparticle size.

In particular, the invention avoids the formation, in spite of the bulkof the total mass treated, of conglomerates of various compositionsresulting from excessive local raising of the temperature. Theseconglomerates otherwise appear inevitably when chromium oxide in a largemass is heat-treated with magnesium vapour.

The invention therefore provides a means for extending the industrialuse of magnesium as a reducing agent for oxides, such as Cr O whoseformation heat is not so great as that of the oxides usually reduced bymagnesium, while keeping the reduced metal in the state of a very finepowder.

In one embodiment of the invention, the mass of chromium oxide to betreated is distributed in crucibles or pots piled one on top of eachother, means being provided for the provision of thermal insulationbetween the successive crucibles.

To provide this thermal insulation, the invention proposes theapplication of an insulating layer as a heat barrier between theportions contained in two successive crucibles. For this purpose itproposes, for example, divided magnesia or alumina in the form of apowder of sulficiently porous agglomerate.

The invention is also characterised by the use of graphite as materialto make the crucibles intended to be superimposed on each other, whichretain their form in spite of the mechanical stresses and thermalinfluences to which they are subjected, so that they can be disposed ina column for the treatment of large masses.

The invention also relates to an arrangement whereby the graphitecrucibles are disposed in several columns; the high conductivity of thegraphite ensures a good distribution in the reaction masses of thethermal flux coming from one or several peripheral resistors.

The invention also proposes placing the reaction mass in iron ormild-steel dishes because of their resistance to corrosion by magnesium,each of the dishes being placed in a graphite crucible.

The crucibles are in an inert atmosphere, for example of argon,leak-proofness being ensured by a cooled seal remote from the treatmentcrucibles.

In the following description given by way of example reference is madeto the accompanying drawings in which:

FIG. 1 is a diagrammatic vertical section through an apparatus accordingto the invention in one embodiment;

FIG. 2 is a diagrammatic section through a crucible according to theinvention in one embodiment;

FIG. 3 is a view similar to FIG. 2 but for another embodiment; and

FIG. 4 is a diagrammatic horizontal section through another embodimentof the apparatus according to the invention.

With reference to FIG. 1, the furnace 10, which is raisable andlowerable, has a wall 11 made of refractory material on whose inner facethere is a heating resistor 12, which is advantageously divided intosections; the current supply of each section may be controlledindependently by means indicated diagrammatically at C. In FIG. 1, thefurnace rests on the bottom 13, thus bounding a cylindrical space 14.The treatment enclosure 15 is bounded by a bell-shaped member 16, madefor example of refractory alloy and of a generally cylindrical shape,leak-proofness at the bottom edge of the ball-shaped member beingprovided by a known seal 17 made of rubher or the like and cooled bywater circulation.

Crucibles or boxes 18 18 18 etc., for example about ten in number, arepiled one on top of the other, the crucible 18 resting by its bottom 19on the top edge 20 of the crucible 18 etc. The bottom crucible 18 restson a base 21, keeping it away from the bottom of the furnace and theseal 17. Each of the crucibles 18 is made of graphite and contains inits interior an iron or mild-steel dish 22.

Heat insulation is provided between two adjacent crucibles. In theembodiment described heat insulation 24 is provided between the bottom19 of each crucible, with the exception of the bottom crucible 18 andthe bottom 23 of the dish it contains. This insulation may consist of alayer of powdered alumina or magnesia. As a modification, a disc may beprovided that is formed by an agglomerate of a material that issufiiciently insensitive to magnesium vapours, such as magnesia oralumina, and has 'sufl'icient porosity to ensure good heat insulation. Aporous-carbon disc may also be used.

A magnesium ingot 25 embedded in chromium oxide powder to be treated 26is placed in each of the receptacles 22. The chromium oxide powder iscommercial powder. Its fineness depends on the fineness of the chromiumpowder it is desired to obtain. The quantity of chromium oxide powdercontained in each crucible may be between 1 and about 10 kilograms, andthe magnesium mass accords with the quantity of chromium oxide powder.

Argon, which forms the protective atmosphere, arrives at the bottom ofthe furnace through the passage 27.

The current supply of each section of the heating resistor 12 iscontrolled independently from a thermostatic element that is sensitiveto the temperature of the reaction mass of the crucible or cruciblesopposite the resistor section and therefore mainly heated by the latter.This being so, provision may be made, for the diiferent crucibles, forthe observance of a predetermined law, for example in stages, of thevariation of the temperature according to the time to which correspondsthe progress desired for the reaction, which starts at about 800 C.

A programmer provides in known manner for the control of the electricitysupply of the various resistor sections taking account of the controlprovided by the thermostatic elements.

During heating, the graphite crucibles maintain their shape withoutcreep in spite of the high temperatures they endure; they may be usedfor a considerable number of treatments.

When submicronic chromium oxide powder is used as starting material,chromium powder is obtained whose particles have an average diameterapproximately between a fraction of a micron and several microns.

In the embodiment shown in FIG. 2, an iron or mildsteel dish 40, thebottom of which 41 is convex, for example spherical, contains thereaction mass and rests on a layer 24 of powdered refractory material.

In the embodiment in FIG. 3, the dish 22, which has a substantiallyplane bottom, rests on a slab 42 made of sintered magnesia and held byan inner rim 43 of a graphite dish 44, the bottom of which thus has alarge circular aperture 45. a

In a variant of the apparatus according to the invention, the pots orcrucibles may be placed side by side but separated thermally from eachother. In this case heating is advantageously effected by the roof, thebottom or the roof and the bottom at the same time.

FIG. 4 relates to an apparatus according to the invention having fourcolumns of crucibles 31 -61 each of which is similar to the onedescribed above with reference to FIG. 1. A central post 32 is provided;it may have partitions distributed regularly in the form of a cross withinwardly curved arms. The post and partitions are made of one or severalmaterials adapted to effect good heat insulation. The crucible columnsare thus thermally separated from each other.

The invention provides for the treatment with magnesium vapours ofchromium oxide powders whose various particles are further away fromeach other than those of an ordinary chromium oxide powder. Thispromotes the action of the magnesium vapour and makes it more regular.

To obtain such a blown or thinned powder, it is proposed to subectordinary chromium oxide powder to turbining in a turbine grinder withouta screen. This aeration operation reduces the apparent density from 20%to 30%. The resulting emulsion of chromium oxide powder in air hasenough stability for it to be used with advantage several hours afterits formation.

What is claimed is:

1. An apparatus for the industrial manufacture of chromium powder byreduction of chromium oxide powder by magnesium in vapor formcomprising: a bell-type enclosure, means for heating said enclosuredisposed externally thereof, a plurality of superimposed crucibles eachadapted to contain a reaction mass of substantially l to 10 kilograms ofchromium powder, and an embedded magnesium ingot, and a layer ofheat-insulating material interposed between the reaction masses of twoadjacent superimposed crucibles.

2. An apparatus as claimed in claim 1 wherein the layer is made of apowder of a refractory heat-insulating material selected from the groupcomprising alumina and magnesia, said powder layer being interposedbetween the bottom of a crucible and the receptacle containing thereaction mass placed in the latter.

3. An apparatus as claimed in claim 2, wherein said crucibles are madeof graphite and said receptacles are made of a metal selected from thegroup comprising iron and mild steel.

4. An apparatus as claimed in claim 1, wherein the bottom of eachcrucible has a large central aperture, and a disc of heat-insulatingmaterial resting upon said aperture.

5. An apparatus as claimed in claim 1, wherein said heating meanscomprises electrical resistors divided into a plurality of groupslocated one above the other and forming separate heating zones, theapparatus further comprising heat-sensing elements opposite saidcrucibles, and means connected to said resistors and to said elementsfor controlling the temperature of said resistors as a function ofsignals delivered by said sensing elements during the reductionreaction.

6. An apparatus as claimed in claim 1, wherein said crucibles are piledone on top of another to form a multiplicity of columns, andheat-insulating means between said crucible columns to insulate the samefrom one another.

7. An apparatus in accordance with claim 6, including a centrallydisposed post, said columns being disposed around said post.

8. Apparatus in accordance with claim 7, wherein said post is shaped forpartially receiving said crucible columns, and is formed of materialwhich thermally insulates the columns with respect to each other.

References Cited UNITED STATES PATENTS 1,829,352 10/1931 Heames 263-482,282,235 5/ 1942 Moberly 266-24 2,661,386 12/1953 Lundgren 13-31 X3,227,798 1/ 1966 Delange 13-22 3,403,213 9/1968 Taylor 266-43 X3,416,779 12/1968 Campbell 266-43 X 5 I. SPENCER OVERHOLSER, PrimaryExaminer JOHN S. BROWN, Assistant Examiner US. Cl. X.R.

